Sensor incorporating a quantum dot as a reference

The present invention provides a device and methods for quantitatively detecting the presence of an analyte by referencing the analyte variant signal to an analyte invariant reference signal. The devices contain quantum dots as the invariant references. The reference quantum dots provide a fluorescent signal whose intensity in invariant with respect to the optical signal associated with the analyte concentration. Both the optical path for the analyte specific and invariant reference signal are substantially identical. Therefore, by measuring the intensity ratio of the analyte specific signal to the reference signal, errors associated with optical path length, absorptivity and scattering are eliminated.